PCAT Chemical Processes: Functional Groups in Organic Chemistry

by Sam

Nov 27, 2017

Chemical Processes, PCAT Blog

Organic chemistry is a major challenge that you’ll face on the PCAT, and the concept of functional groups lies at the core of organic chemistry. There’s no way around the fact that you have to memorize a lot of information to succeed with organic chemistry on Test Day, but developing a thorough knowledge of functional groups and an understanding of why they behave the way they do will both help you approach organic chemistry on the PCAT and serve you well in your future career as a pharmacist.

Functional groups are groups of atoms that behave in predictable ways when embedded in various molecules. They dictate how a compound behaves both in terms of physical properties and chemical properties and reactivity.

In this blog post, we’ll work through a few key principles underlying why functional groups act the way they do:

First, physical properties such as melting point and boiling point are largely shaped by intermolecular forces, which in turn are primarily driven by charge-based interactions. Hydrogen bonding, which refers to strong dipole-dipole interactions between a partially positive H atom and a lone pair on a highly electronegative atom like O, F, or N, is a particularly strong covalent intermolecular force that is associated with elevated melting points and boiling points. Water is a classic example of a molecule whose behavior is shaped by hydrogen bonding, as well as alcohols (–OH) and carboxylic acids (–COOH). Primary and secondary amines (–NH2 and –NH) can also engage in hydrogen bonding. Dipole-dipole interactions occur between molecules with polar functional groups that cannot engage in hydrogen bonding. The classic example are ketones and aldehydes, with C=O bonds. These forces are weaker than hydrogen bonds, but still elevate the melting and boiling points in comparison to nonpolar molecules, such as hydrocarbons, which experience London dispersion forces.

Charge distribution also shapes reactivity. In organic reactions, a nucleophile generally reacts with an electrophile, and identifying which atom is which can help you understand reaction mechanisms and predict the outcomes. Nucleophiles have excess electrons—implying a partial negative charge—and “want” to use those electrons to form a bond. Oxygen in organic compounds is a classic example, because it generally has two lone pairs. In contrast, electrophiles “want” electrons, because they tend to be partially positively charged. Carbonyl (C=O) carbons are a classic example of this, and reactions involving a nucleophilic attack on an electrophilic carbonyl carbon—as in aldehydes, ketones, or even carboxylic acids—are a PCAT mainstay.

Let’s not forget resonance! Resonance takes place when multiple Lewis dot structures can be drawn for a molecule. The actual underlying structure of a molecule can be thought of as a blend of its resonance structures, and on an even deeper level, resonance generally involves electron destabilization. The takeaway point for the PCAT is that resonance can help stabilize a molecule by delocalizing charges. A classic example is that resonance stabilizes the conjugate bases (–COO–) of carboxylic acids, which is why carboxylic acids are relatively acidic. Resonance also helps explain the stability of peptide bonds, which are the bonds that hold protein chains together.

A lot more could be said about functional groups and organic chemistry, but hopefully this blog post has pointed you in some useful directions to help organize your knowledge. Remember to always try to understand why in organic chemistry, instead of just memorizing inputs and outputs.